1. Field of the Invention
This invention relates to antimicrobial compositions comprising a mixture of hinokitiol and citronellic acid in a specific ratio. The antimicrobial compositions according to the invention are safe for humans, have a high antimicrobial activity and a broad antimicrobial spectrum, and can be widely employed in toiletries and household articles, which need antiblastic or antifungal properties.
2. Description of the Prior Art
Many antimicrobial substances are conventionally known which are chemically synthesized or derived from natural substances. Some of them are officially described in the Official Formulary of Food Additives, The Japanese Standards of Cosmetic Ingredients, etc., and are employed in toiletries and household articles in order to prevent infections by bacteria and fungi. Recently, antimicrobial ingredients in food additives and cosmetic ingredients have been needed for their safety for humans. Therefore, ingredients derived from natural substances are attracting attention because they are said to be very safe.
However, these naturally-occurring antimicrobial substances often have a narrow antimicrobial spectrum, and sometimes are not adequately applicable for toiletries and household articles because they do not meet the requirements of a broad antimicrobial spectrum against various kinds of microorganisms. For example, antimicrobial substances derived from essential oils of spices and herbs have high antimicrobial activity against fungi, but not against bacteria. Antimicrobial substances derived from essential oils of eucalyptus, cinnamon, cedar, sandalwood, etc. are effective against bacteria but not fungi.
Hinokitiol is an effective antimicrobial and is also called .beta.-thujaplicin which is derived from essential oil of Chamaecyparis taiwanesis and Thujopsis dolabrata, etc., and is known to be one of the compounds having the highest antimicrobial activity and the broadest antimicrobial spectrum among known naturally occurring antimicrobial substances. Hinokitiol has a minimum inhibitory concentration (MIC) of about 1/20000 (50 ppm) against Eshcerichia coli, while it has a MIC of about 1/10000 (100 ppm) against Pseudomonas. Hinokitiol is reported to have a low antimicrobial activity against other microorganism as well as Pseudomonas (Yuzo Yamaguchi, Fragrance Journal, No. 46 (1981) pp. 56-59).
An antimicrobial agent comprising hinokitiol is disclosed in JP-A-4-182408 (1992) (The term "JP-A" as used herein means an "unexamined published Japanese patent application"). The agent is a fungicide, and is applicable for use on turf, etc., to prevent diseases by inhibiting mold (fungus).
Since hinokitiol has a strong odor, it can be added to toiletries and household articles only in low concentrations so that the effect of the odor is minimized. However, these amounts of hinokitiol are not sufficient to inhibit growth of bacteria belonging to the genus Pseudomonas and mold.
Recently, hinokitiol compositions have been prepared which are combined with other substances to elevate the antimicrobial activity of hinokitiol. For example, JP-A-2-243607 (1990) discloses that a combination of hinokitiol and phenoxyethanol has elevated antimicrobial activity against Pseudomonas aerginosa, Eshcerichia coli, Staphylococcus aureus, Bacillus subtilis, Candida albicans, and Aspergillus niger, which is applicable as antiseptics for toiletries. Also, JP-A-5-271073 (1993) discloses a combination of hinokitiol and indole as an antimicrobial agent which has an elevated antimicrobial activity against Pseudomonas aeruginosa.
Citronellic acid is also called 1-rhodinic acid. It is a compound derived from an essential oil of Chamaecyparis taiwanesis, etc., and it is known to have an antimicrobial activity against Fones annosus and Mycobacterium tuberculosis (Osamu Okuda, Koryo Kagaku Soran (Fragrance Chemistry Comprehensive Bibliography) [II], 1968. 1.15., issued by Hirokawa Shoten, p. 1140). Also, JP-A-6-40831 (1994) discloses that a plant component derived from Cupressaceae contains .beta.-dolabrin and carvacrol in addition to hinokitiol and 1-rhodinic acid (citronellic acid) and has an antifungal activity against pathogens which cause turf diseases.
JP-B-1-40005 (1989) (The term "JP-B" as used herein means an "examined published Japanese patent application") discloses anti-dental caries compositions comprising hinokitiol and carboxylic acids which have antimicrobial activity against Streptococcus mutans causing dental caries. 1-Rhodinic acid (citronellic acid) is described therein as an example of carboxylic acids. However, the antimicrobial activity of compositions comprising hinokitiol and citronellic acid is not definitely described. Although straight chain carboxylic acids such as myristic acid, lauric acid, and oleic acid are described in combination with hinokitiol, the ratio of hinokitiol and carboxylic acid ranges from 1:4 to 1:20 by weight.
As described above, hinokitiol compositions have been described in the art which further include ingredients to increase the antimicrobial activity of hinokitiol. However, several problems are raised: their safety for humans is not guaranteed because the ingredients combined with hinokitiol are not derived from natural substances, use of the compositions is restricted because they provide only a narrow spectrum of activity against microorganisms, etc. Although combinations of hinokitiol and citronellic acid are known, compositions of hinokitiol and citronellic acid in known ratios are not satisfactory because the antimicrobial activity of these compositions is not sufficiently high, they are effective against only a restricted antimicrobial spectrum of microorganisms and their applications are also restricted. Further, no description or suggestion is present in this art on excellent antimicrobial activity of the compositions comprising a specific ratio of the ingredients which has now been found.